1. Field of the Invention
Embodiments of the invention generally relate to fiber optic cable systems for use in harsh environments such as gas and oil wellbore applications.
2. Background of the Related Art
With advancements in the area of fiber optic sensors for use in harsh environments, there is an increasing need for fiber optic cables compatible with the harsh environmental conditions present in oil and gas wellbore applications. For example, fiber optic cables utilized in sensing applications within the wellbore must be able to operate reliably in conditions that may include temperatures in excess of 300 degrees Celsius, static pressures in excess of 138,000 kilopascal (kPa), vibration, corrosive chemistry and the presence of high partial pressures of hydrogen. The hydrogen tends to darken waveguides in the cable causing undesired attenuation.
FIG. 7 depicts one example of a conventional fiber optic cable 700 suitable for use in harsh environments such as oil and gas wellbore applications. The fiber optic cable 700, shown in FIG. 7, includes a fiber in metal tube (FIMT) core 702 surrounded by an outer protective sleeve 704. The FIMT core 702 includes an inner tube 706 surrounding one or more optical fibers 708. Three optical fibers 708 are shown disposed within the inner tube 706 in the embodiment of FIG. 7. A filler material 710 is disposed in the inner tube 706 to fill the void spaces not occupied by the optical fibers 708. The filler material 710 may also include a hydrogen absorbing/scavenging material to minimize the effects of hydrogen on the optical performance of the fiber 708. The outer protective sleeve 704 includes a buffer material 712 and an outer tube 714. The buffer material 712 provides a mechanical link between the inner tube 706 and the outer tube 714 to prevent the inner tube 706 from sliding within the outer tube 714. Additionally, the buffer material 712 keeps the inner tube 706 generally centered within the outer tube 714 and protects the inner tube 706 and coatings formed thereon from damage due to vibrating against the outer tube 714.
At least one of the inner or outer surfaces of the inner tube 706 is coated or plated with a low hydrogen permeability material 716 to minimize hydrogen diffusion into an area around the optical fibers 708. As temperature increases, materials (e.g., the low hydrogen permeability material 716) of prior cables disposed around optical waveguides to provide hydrogen blocking become less effective since hydrogen diffuses faster through these materials. This susceptibility of the optical waveguides to attack by hydrogen in high temperature environments reduces service life of the cables.
Therefore, there exists a need for improved fiber optic cables and methods for use in harsh environments.